Numerous documents describe devices and methods for testing the eyes of a person.
Some devices and methods for testing the eyes are of subjective type, because they require the active assistance of the person. Other devices and methods for testing the eyes are of objective type, because they are independent of the person's perceptions or reactions.
The objective type devices for testing the eyes refraction usually require large, heavy and expensive equipment, such as an autorefractor. Generally, an objective type device needs power supply, is not very reliable or requires the intervention of a qualified person.
Subjective testing can provide precise optometric measurements. However, a normal refraction measurement process is complex and time-consuming (more than 15 minutes per tested person) and requires the intervention of an optometrist and/or a medical specialist.
Subjective devices such as screening charts also exist that are less sophisticated equipment and can be used on a larger basis. As an example, Snellen's, EDTRS or Bailey Lovie acuity charts are well known subjective type devices for testing the eyes of a person. These charts display lines of target alphanumerical characters printed in sharp block letters and placed in lines of decreasing size. A person subject to a visual test is placed at a distance from the chart and reads the target alphanumerical characters from the larger size down to the smallest readable line. The distance between the chart and the person is generally set to more than 3-4 meters for far vision test and to less than 60 cm for near vision test. A scale, displayed at the end of each line, indicates the corresponding visual acuity. The smallest line that can be read by the person accurately indicates the visual acuity of this person, giving thus an evaluation of the refraction correction needed by this person.
This visual test can be performed in monocular or binocular vision conditions, for near vision or far vision test, with or without corrective lenses. These charts require only a printed board and can be used for evaluating the visual acuity of any person able to read the alphanumerical characters displayed.
Other charts dedicated to young children or illiterate persons are based on other target optotypes such as images or symbols that do not require deciphering alphanumerical characters. In all cases, the target optotypes are displayed with a high clearness and contrast (in general black optotypes on a clear background).
However, without the use of any corrective lenses, the estimation of the correction needed is very poor and does not give the type of correction needed (spherical or astigmatism). The use of corrective trial lenses can provide precise optometric measurements. However, a normal refraction measurement process is complex and time-consuming (more than 15 minutes per tested person), requires the use of a full and expensive set of trial lenses and requires the intervention of a skilled person such as an optometrist and/or a medical specialist.
A simple astigmatic test chart, called Parent Dial, allows screening for a particular orientation of an axis of astigmatism. This astigmatic test chart displays a set of line segments disposed radially around a center point, with a constant angular pitch and displays the corresponding angles from 0 to 180 degrees every 10 or 20 degrees, like a protractor. A person subject to a visual test is placed at a distance from the Parent Dial. The target lines are displayed with a high contrast and clearness. However, when the eye of the person presents some degree of astigmatism, some lines are seen sharp and other lines are seen blurred by this person. The person subject to the test indicates the sharpest line seen which corresponds to the orientation of an axis of astigmatism for the tested eye of this person. However, an astigmatic test chart is not very reliable and does not enable, alone, determining the cylindrical degree of correction needed.
Based on an astigmatic test chart, a method for evaluating the cylindrical degree of correction has been developed. The axial angle of astigmatism being determined, a cylindrical lens is inserted on the optical axis between the tested eye and the test chart, the cylindrical axis of this lens being set orthogonal with the axial direction seen clearly. The value of the cylindrical degree C. of the cylindrical lens is adjusted by changing the cylindrical lens power, until the astigmatic test chart is sharply seen uniformly. However, this astigmatic test chart and method requires a set of cylindrical lenses of different optical powers and requires the intervention of a skilled person such as an optometrist and/or a medical specialist.
The patent document EP 1250883_B1 discloses an optometry apparatus comprising a light source, a target object, spherical and/or cylindrical optical lenses and a diffraction grating plate forming different target images that appear to the examined eye(s) dispersed in a plane orthogonal to an optical axis and shown simultaneously at different virtual distances in the direction of the optical axis. However, this system is complex and needs a power supply for the light source.
Another subjective type device is based on a volume phase hologram (Avudainayagam K V, Avudainayagam C S, Nguyen N, Chiam K W, Truong C., J Opt. Soc. Am. A Opt. Image Sci. Vis. 2007 October; 24(10):3037-44, “Performance of the holographic multivergence target in the subjective measurement of spherical refractive error and amplitude of accommodation of the human eye”). A complex wavefront arising from illuminating different targets placed at different distances is recorded in a single volume phase hologram. When this hologram is illuminated by a phase conjugated beam, each target forms a three-dimension image having a different vergence in a range from +5 to −2.5 diopters and in steps of 0.5 diopter. From the image target seen clearly by the subject, and the corresponding vergence, the spherical refractive error and the amplitude of accommodation of the human eye can be evaluated. However, this hologram requires a complex illuminating system or setup and can be seen only by one person at a time.